1. Field of the Invention
The present invention relates to a gas sensor, an electrically conductive oxide sintered body for use in a gas sensor, an electrically conductive oxide sintered body, and a wiring board.
2. Description of the Related Art
Generally, ceramic electronic devices have a ceramic substrate and electrodes disposed on the substrate, and such electrodes are formed of a metallic material. Such ceramic electronic devices include, for example, multi-layer ceramic capacitors equipped with an Ni electrode, a Pd electrode, or a Pt electrode; low temperature co-fired ceramic (LTCC) parts equipped with an Ag electrode, a Cu electrode, or an Ag—Pd electrode; piezo-actuators having a Pd electrode; semiconductor packages employing a W electrode; and spark plugs having an Ir electrode or a Pt electrode.
Among the above metal elements, Ni, Cu, and W must be fired with a ceramic substrate in a controlled atmosphere. Thus, difficulty is encountered in attaining characteristics intrinsic to the target ceramic substrate, and production costs rise, which is problematic. In the case of Ag, which has a low melting point (962° C.), the material of the ceramic substrate is limited. In addition, when the substrate is fired at a low temperature, the characteristics of the ceramic substrate may be impaired. Noble metals such as Pd, Ir, and Pt are expensive, making application of such materials to large area electrodes difficult.
Patent Document 1 discloses, as an oxide electrode material, a lanthanum cobalt oxide having a negative temperature characteristic such that the resistance thereof is high at ambient temperature and lowers with rising temperatures. Patent Document 2 discloses a lanthanum cobalt oxide having a negative temperature characteristic such that the resistance thereof is high at room temperature and the absolute value of its B constant is large at a high temperature. However, the conductive oxides disclosed in Patent Documents 1 and 2 exhibit high resistivity, i.e., poor conductivity, at room temperature.
As described above, conventional oxides have a conductivity which is considerably lower than that of metallic materials, and a large absolute B constant value (temperature coefficient). Therefore, difficulty is encountered in replacing a metal electrode with a ceramic electrode. Meanwhile, ruthenium oxides (e.g., RuO2 and SrRuO3) are known to have high conductivity, but are problematically expensive. Under such circumstances, the present applicant discloses in Patent Document 3 an oxide sintered body which has a high conductivity and a small absolute B constant value (temperature coefficient) and which is suited for a conductive material. Also, Non-Patent Document 1 discloses various types of perovskite oxides. Patent Documents 4 and 5 disclose lanthanum-nickel-copper-iron perovskite oxides ((La, Ni)(Cu, Fe)O3). The oxides disclosed in Patent Documents 4 and 5 are used as, for example, an electrode oxide material for solid oxide fuel cells.